Physics & Astronomy ETDs

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The present work is concerned with the mean lifetime of compound nuclei formed in the fast-neutron-induced-fission process. A new method for the measurement of these lifetimes is proposed. The present method is based on the recoil of the compound nucleus following neutron capture and the secondary electrons emitted by fission fragments. Previous experimental and theoretical work concerning the secondary electron yields for fission fragments and other charged particles are reviewed. The importance of the higher energy electrons, the so-called delta rays, formed in the primary ionization process is considered. The subsequent secondary ionization produced by these delta rays provirles a natural explanatior of a previous experimental fact that the number of electrons emitted by a fission fragment is larger when the fragment emerges than when it enters the same surface. This viewpoint is extended t0 a consideration of the dependence of the electron yield on the depth beneath the surface at which a fission event takes place. On the basis of a qualitative theory, this yield is expected to increase with depth and approach asymptotically a limiting value which corresponds to the case where the fragment emerges from depths larger than the maximum range of the delta rays. Measurements of this dependence of the yield on the depth were performed with layers of uranium tetrafluoride. In these measurements iv the electrons were detected by accelerating and focusing them on a scintillation detector. A measure of the number of electrons emitted by each fragment was obtained from the height of the pulse from this detector. The general dependence predicted by the theory was observed. These results were used in an attempt to measure the average distance traversed by the recoiling compound nuclei during the time interval between neutron capture and fission. In one arrangement of the apparatus the compound nuclei with a given recoil velocity were expected to move away from the foil surface a distance proportional to their mean lifetime. Later, the arrangement of the apparatus was changed so that the nuclei were expected to move close to the surface. Measurements of the average electron yield were in each position. From a comparison of these results and the previously determined dependence of the yield on depth, an estimate of the average recoil distance and, therefore, a measure of the mean lifetime could be obtained. Target nuclei of uranium-238 and neptunium-237 were used in this experiment. Neutrons with an assumed fission spectrum were obtained from a reactor. The results of these measurements showed that the mean lifetimes of the compound nuclei of U-239 and Np-238 were, within the sensitivity of the present method, consistent with zero. Upper limits of the lifetimes have been obtained, however, which are considerably lower than those previously obtained. The total mean lifetimes for target nuclei of U-238 and Np-237 were found to be T(U-238)<6 x 10-14 sec and T(Np-237)<4 x 10-14 sec. The partial mean lifetimes for fission and neutron emission were determined from the relation r = rf + rn and the V previously measured ratio of the fission and neutron emission widths. The partial mean lifetimes are as follows: Tf(U-238) < 4 x 10-13 sec; Tn(U-238) < 7 x 10-14 sec; Tf(Np-237) < 9 x 10-14 sec; and Tn(Np--237) < 7 x 10-14 sec.

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Physics & Astronomy

First Committee Member (Chair)

Christopher Pratt Leavitt

Second Committee Member

John Root Green

Third Committee Member

Roy Thomas



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